This invention relates in general to measurement apparatus and method; more particularly, to a system and method for analyzing components of a signal; and more particularly still, to a system and method for analyzing jitter in a serial data signal.
Jitter in serial data communication is a difference of data transition times relative to ideal bit clock active transition times. Jitter represents a deviation, typically in picoseconds, from the ideal. As data transfer rates increase within semiconductor devices and other high speed applications increase, the jitter component may become more significant. For example, in video graphics chips, jitter can cause a flicker or jumping of the video image. Also, in serial data communication systems jitter will cause errors. In order to understand the effects that jitter may have on semiconductor devices and data communication systems, measurements of jitter and other aspects of timing are critical during the prototyping stages and production tests.
Jitter has four major components, inter-symbol interference (ISI), Duty Cycle Distortion (DCD), periodic jitter (PJ), and random jitter (RJ). ISI is caused by a data path propagation delay that is a function of past data history and occurs in all finite bandwidth data paths. DCD is caused by differing propagation delays for positive and negative data transitions. PJ is caused by one or more sine waves and its/their harmonics. RJ is assumed to be Gaussian (normal) and has a power spectral density that is a function of frequency.
ISI, DCD and PJ are all bounded. They may be described as a peak or peak to peak value in bit period, unit interval (UI), or seconds. PJ in general has a magnitude for each spectral line.
RJ is unbounded. It may be described by a standard deviation in UIs or seconds. In production testing a jitter tolerance mask is used to mimic the behavior of an actual data communications receiver in that it rejects low frequency jitter. A jitter tolerance mask is a function that is defined in the frequency domain. It has a magnitude that is a function of frequency. In most serial data communications systems, the data/clock recovery circuit tolerates low frequency jitter more than high frequency jitter and the shape of the mask reflects this fact.
An analysis of the jitter components is valuable to the product designer and tester. For example, measuring PJ helps determine whether there is cross-talk on a circuit. Analyzing ISI and DCD permits the cause of the bit error rate to be determined.
The present available measurement instruments, however, do not separate the jitter components. For example, oscilloscopes show jitter as an overall distribution without a separation of RJ and PJ. Although spectrum analyzers may measure jitter, they typically cannot be used on data streams. Moreover, when a spectrum analyzer measures a jitter value it does not separate PJ and RJ.
Bit error test equipment allows a serial data pattern to be compared with a test pattern. Although a bit error rate is determined, no information is provided about any of the jitter components of the bit error rate. The relative proportions of the components is undetermined. Also, bit error test equipment provides a slow method of estimating bit error rate. Two hours is typically necessary to estimate the bit error rate with statistical confidence for a bit rate of one gb/s.
The present invention provides a solution to this and other problems, and offers other advantages. Indeed, various embodiments construed in accordance with the principles of the present invention include the advantages of measuring jitter components without the need of a bit clock, performing statistical checks on measurements, and generating a statistical prediction of the failure rate.
The present invention discloses a method, apparatus, and article of manufacture for analyzing jitter. Inter-symbol interference, duty cycle distortion, random jitter and periodic jitter are measured. The method includes the steps of obtaining measurements of the spans of a signal, generating variation measurements for each of the spans, transforming the variation estimates from a time domain to a frequency domain, and determining the random component and the periodic component of the jitter signal.
Another embodiment provides for the measurement of ISI, DCD, RJ and PJ in serial data communications. The method includes the steps of measuring a unit interval of the data pattern, determining a matching pattern, calculating a measurement schedule defining a measurement set for each span, obtaining a plurality of measurements for many of the spans, generating variation estimates each of the spans, transforming the variation estimates from a time domain to a frequency domain, and determining the random component and the periodic component of the signal. The random and periodic components are determined through the application of a constant false alarm filter.
According to another aspect of the invention, there is provided an apparatus for measuring jitter in a signal having a repetitive data pattern. The apparatus comprises a measurement apparatus for collecting data and an analyzing unit for analyzing the signal and determining the random component and the periodic component of the signal.
According to yet another aspect of the invention, there is provided an article of manufacture comprising a program storage medium for storing instructions to execute method steps for analyzing jitter in a signal and for determining the random component and the periodic component of the signal.